Recent years have seen growing cable televsion industry interest in various types of off-premises signal control systems, such as interdiction systems for denying programming to cable or community antenna television (CATV) subscribers who have not purchased it. A number of concerns with such off-premises equipment revolve around the need for providing electrical power to the circuit. One concern is the cost burden to the cable system operator if the off-premises system is powered from the feeder or distribution cable, or the cost burden to the subscriber if powered from the subscriber's facility. Another concern is the compatibility of an off-premises system into an existing plant, since a retrofit of new equipment such as an interdiction system must be virtually transparent to subscribers. A third and primary concern is safety if power is being supplied from the subscriber's facility so as to ensure that no shock hazard will be created. The latter concern is especially important in cable television distribution systems for home viewing, since cable subscribers often connect and disconnect their own cable equipment such as VCR's, cable converters, and the like.
Because of the increasing popularity and acceptance by cable system operators of signal control systems such as interdiction systems, there is a particular need for power circuits for powering off-premises cable television channel interdiction systems such as those shown in U.S. Pat. No. 4,912,760 to West, Jr. et al., and U.S. Pat. No. 5,109,286 to West, Jr. et al., both owned by the assignee of the present application. In these systems, an off-premises cable television interdiction apparatus comprises a microprocessor actuation and control means for actuating and controlling a plurality of frequency agile voltage controlled oscillators. The voltage controlled oscillators selectively jam only unauthorized premium programming transmitted "in the clear" from a headend system to a particular subscriber. The interdiction system prevents the reception of jammed premium programming by the unauthorized subscriber. During the normal mode of operation, a frequency hopping circuit operates to receive and store voltage control words for operating the oscillators consistent with a headend selected jamming factor for a particular channel to be jammed and addressably transmitted and stored premium programming authorization data.
Off-premises interdiction and other signal control devices contain electrical devices for receipt, processing, and storage of control information signals from the headend as well as for re-transmission or jamming of the programming signal, and require electrical power for operation. The devices present to be powered typically include at least one microprocessor, associated memory, voltage controlled oscillators, broadband amplifiers, video amplifiers, and other electrical components. These circuits can draw as much as about 25 watts of power for each subscriber. Although 25 watts per subscriber is a relatively nominal power drain if taken alone, or if provided by the individual subscribers, the aggregate power requirements for a large cable system would nevertheless constitute a significant cost factor to the operator if the entire system were required to provide power for the interdiction systems for all subscribers.
Some off-premises signal control systems have power consumptions per subscriber of about 4 watts for interdiction systems and up to about 35 watts for frequency (channel) conversion systems. For a system designed for about eighty subscribers per mile, the signal control power consumption would be 320 watts per mile for an interdiction system, and 2,800 watts per mile for a frequency conversion system. It will thus be understood that the power requirements for the cable operator can be substantial in a large system. Additionally, it is known that approximately 15-20 percent of power supplied is actually lost as heat in trunk and feeder cables. For example, in carrying 10 amps through one mile of 3/4 inch copper clad aluminum center conductor GID cable, 380 watts are dissipated. For smaller cables, such as 1/2 inch cable, up to 866 watts is lost.
When off-premises signal control powering needs are considered, and taking into account factors for cable loss and power supply efficiencies, usage of interdiction systems and conversion systems adds to a power cost for signal security alone of in excess of $100,000 per year for a 100 mile system, and such calculations do not take into account the capital and labor cost for installation. Accordingly, there is substantial interest in providing power for off-premises signal control systems by subscriber located power supplies.
It is believed that, if power supplies for interdiction and similar signal control systems can be made small and unobtrusive, and if the power consumption can be held sufficiently small to be insignificant compared to the other power usage of the subscriber, there will be fairly good acceptance of subscriber-located power supplies. Typical cable subscriber drops are RG-59 size foam dielectric cable with an aluminum foil sheath and a copper clad steel center conductor of 73 ohms per 1,000 feet loop resistance. Provision of a 3 watt interdiction system would cause a 0.95 volt drop in the drop cable, with 3.12 watts being supplied by the subscriber. If an 80% efficiency of the voltage conversion in the subscriber's home is assumed, the interdiction system would draw 3.9 watts from 120 volts alternating current (VAC), at a nominal cost of about $2.75 per year at current power rates in many parts of the United States. This amount of power consumption is considered nominal and should be accepted by most subscribers by explaining that the subscriber's fees would have to be increased if the subscriber were not supplying the power.
If power is to be supplied at the subscriber's facility, safety concerns become paramount. It is generally considered that 24 volts drawn from an AC to DC power converter is considered safe in an indoor environment. However, since it is possible that a small child would pick up a broken drop cable while standing in a puddle of water, additional safety requirements are present. It is known that the most hazardous frequency for cardiac fibrillation and the primary cause of death from low voltage electrical shock is close to about 60 Hz, the common AC power supply source in the United States. It is therefore important that the power supply avoid frequencies at or near 60 Hz.
It is also known that if the frequency could be reduced or eliminated to DC, or to a low frequency alternating current of about a few tenths of a Hertz to avoid galvanic corrosion problems, or the AC frequency could be increased to approximately 10 kHz, the safety factor is substantial greater. The provision of electrical power by pure DC is not considered acceptable because of galvanic corrosion problems. Moreover, the provision of power at frequencies approaching or exceeding 10 kHz are undesirable due to the additional expense and complexity of the circuitry and the need to block the 10 kHz and harmonics from the information signal.
One prior art approach is the provision of off-premises power supply circuitry that interrupts the voltage leaving an in-premises power supply if the drop current is interrupted. In this particular system, when drop integrity is restored, the system automatically restarts if some other subscriber or subscribers are still supplying power to the off-premises device. If that subscriber was the only one supplying power, he may attempt to restart by pressing an "on" button. Power will come on for a predetermined time period of several hundred milliseconds, and if no data communications are established from the off-premises device within that time, the system assumes drop integrity has still not been restored and voltage is again interrupted. This timing is considered sufficiently shorter than the time required for a shock hazard to be generated at the voltages involved.
It is known that powering may be accomplished for off-premises equipment by either AC or DC. AC is generally favored by the CATV operator's community due to fewer problems with corrosion as a result of electrolysis. However, use of AC presents difficulties of successfully powering the equipment while meeting applicable regulatory and safety requirements. Use of DC would overcome the regulatory and safety difficulties, but presents a corrosion difficulty.
In a typical CATV subscriber connection, electrical power must pass through at least four electromechanical connections--one at the power source, two at a ground block just outside the subscriber's premises, and one at the off-premises equipment. Each of these connections is typically made with the F-connector commonly used in the CATV industry, which is known to be susceptible to corrosion in the presence of direct current. The corrosion effect is explained by Faraday's principles of electrolysis, that the quantity of an element undergoing chemical reaction at an electrode is proportional to the quantity of electric charge passing, proportional to the atomic mass of the element, and inversely proportional to its valence. It is known to those skilled in the art that electrodes are formed at each electromechanical connection in which air can be in proximity to the metal. Since air carries moisture, the classic conditions for electrolysis are met--an electrolyte (moisture in the air combined with surface impurities), dissimilar metals in the mechanical mating of two surfaces at the connector, and current (for the instant case of powering through the coaxial cable). Under these conditions, the elements on each of the mating connector parts will undergo a chemical reaction. This chemical reaction will ultimately result in the removal of material from one contact in such a way that, after sufficient time has passed, the electrical contact between the two connectors fails. The result is the undesirable and annoying interruption of cable television service to the subscriber.
Accordingly, there is a need for a power supply circuit for remotely located off-premises signal control systems which provides the safety advantages of DC, but avoids the electrolysis problems of DC.